Question: Which of these numbers is prime? ${14,\ 43,\ 75,\ 81,\ 85}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 14 are 1, 2, 7, and 14. The factors of 43 are 1 and 43. The factors of 75 are 1, 3, 5, 15, 25, and 75. The factors of 81 are 1, 3, 9, 27, and 81. The factors of 85 are 1, 5, 17, and 85. Thus, 43 is a prime number.